1. Technical Field
This invention relates generally to a method and apparatus for operating lamps, and more particularly to lamp controllers for preserving the performance characteristics of discharge lamps.
2. Description of the Related Art
High performance lighting systems are known in the art which incorporate discharge lamps designed for producing bright light from a small arc, and precision reflectors. Such systems include HID lamps that are short-arc high pressure mercury lamps in the form of a spherical or ovoid quartz bulb with a very short gap between the electrodes, typically between 1 mm and 1.4 mm. As a result, arc luminances rank among the highest of all lamp types. The chief asset of the short-arc mercury lamp is that it forms a near point source of light of very high luminance which makes it ideal for systems that employ a critically focused optical light collection system which usually depends on the maintenance of a very small arc length and accurate arc position for optimum performance. This makes it ideally suited for projection purposes, for example, a TV projection system or an LCD display apparatus.
These lamps are used in connection with precision reflectors and optics to produce a uniform collimated output with maximum light collection. The imaging properties of these optics are not critical, but rather their function is to collect substantially most of the energy emitted from the arc lamp and direct it into a beam with minimum etendue or geometric optical size. It is advantageous for the optical collection system to produce a light beam with as small an extent as possible to reduce power consumption and to reduce manufacturing costs associated with projection systems of smaller size and/or aperture.
There are several advantages associated with this system. As described above, it is desirable to produce light from a small arc gap because, inter alia, the smaller the arc gap the smaller the power that is needed to produce a given light output. Likewise, the optical collection systems described above incorporate reflectors that require a stable arc, and whose collection efficiency is severely reduced when the arc moves about significantly. For example, in some systems, a shift in the arc of only 1 mm can result in loss of up to 50% of collected light.
Inherent characteristics of the above lamp are variations in the arc gap between the electrodes. During AC operation of the lamp, the discharge arc is not stable because the origin of the discharge arcs jumps among random locations at the tip of the electrode. Likewise, tungsten material (W) from the electrode itself evaporates during lamp operation and redeposits on the electrode tip.
It is known in the art to provide arc lamp power supplies which are configured to produce a constant power for the arc lamp. When such supplies are employed, power within the arc is measured, and the current is adjusted such that a desired power is achieved and maintained. Consequently, if the arc burns back the electrodes to produce a longer arc, or the arc for any reason becomes shorter, appropriate voltage and current changes are invoked to maintain the preset power desired. However, these power supplies have no mechanism for sensing or control of the lamp arc gap. As a result, the arc gap is left to find its own natural dimensional envelope while power is maintained. Thus the arc gap may fluctuate or reach unacceptable dimensions.
Several approaches have been tried to improve the consistency and reliability of lamp operation. For example, U.S. Pat. No. 5,608,294 to Derra et al, which is hereby incorporated by reference, discloses a method of operating a lamp to reduce flicker arising from the above-described random jumping of the discharge arc. The method disclosed therein comprises the addition of a stabilization current pulse which is generated in a predetermined fraction of the half periods of the AC square wave lamp current. The stabilization current pulse has the same polarity as the lamp current and is superimposed on the lamp current in the latter part of the half period in which it is generated. It was found that when operating a high pressure discharge lamp according to the Derra et al method and apparatus, flickering of the lamp can be substantially suppressed. However, this technique permits the arc gap to vary over an unacceptable range for some critical systems, such as the reflectors and optics described above.